During the manufacture of custom integrated circuits, it may be required that deep holes be fully filled with photoresist. Maintaining photoresist thickness and uniformity under these conditions often presents a major challenge. In particular, if it is necessary to include, as part of the photoresist image, a feature (such as a hole or an island) that is located within about 25 microns from the edge of a cavity, conventional methods for photoresist coating suffer from three serious problems:
1. Because of its viscosity, the photoresist is unable to completely fill deep holes.
2. Bubbles may get trapped within the photoresist used to fill the holes subsequently leading to cracks generated during routine baking of the resist.
3. Photoresist uniformity is poor close to a high topography border, in particular a cavity in the form of a trench or a via hole or a channel (e.g. for conveying fluid),
Referring now to FIG. 1, seen there is a substrate which might typically be made up of a silicon base 11 on whose surface an integrated circuit had been formed. Wiring 14 for said circuit is seen in cross-section as embedded in inter-dielectric layer 13. At its topmost level is passivation layer 15 which covers the highest wiring level. A cavity has been formed that extends downwards from layer 15 well into the silicon.
Using routine prior art methods, photoresist layer 16 has been spread over the surface with the intention of filling any cavities as well. As shown schematically in FIG. 1, layer 16 does not always fill the cavity fully and uniformly. Instead, air bubble 12 gets trapped within the cavity.
FIG. 2 schematically shows photoresist layer 16 being exposed through mask 21 that includes feature 22 which, after processing, leads to openings 31 in the resist directly above wiring 14, as seen in FIG. 3.
Following resist development, the structure gets baked in order to dry and harden the resist. One side effect of this heat treatment is the expansion of the trapped bubble to become bubble 42, as seen in FIG. 4. The upward movement of resist 16 to a new peak height 43 becomes permanent because of the resist hardening so, even after cooling, openings 31 have become distorted and uneven so that subsequent attempts to etch through them, or to use them as molds, produce poor results.
The present invention teaches how this problem can be overcome.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,326,296 B1, Tsai et al. show a process where a dual damascene hole is filled with photoresist by using a protective material to protect the side of the hole. Oshaki et al. in U.S. Pat. No. 5,677,243 reveal a process where an organic material is used as a temporary filling in a dual damascene process. U.S. Pat. No. 6,042,999 (Lin et al.) teaches a process where a resist fills the trench hole as a protective measure.